1. Field of the Invention
The present invention relates to a single crystal growing apparatus suitable for use in pulling a crystal from a melt of a compound semiconductor of elements of Groups III-V, especially gallium arsenide (GaAs), according to the Czochralski method.
2. Description of the Prior Art
For compound semiconductors of the type noted to fully exhibit their intrinsic optical and electrical characteristics, it is necessary that the single crystals prepared have a high purity and a high perfectness of crystal.
Compound semiconductors of elements of Groups III-V such as GaAs, gallium phosphide (GaP) and indium phosphide (InP) melt as high as 1238.degree. C, and 1050.degree. C. and exhibit high dissociation pressures (0.9, 30 and 15.5 atm respectively) at such melting points. Growing these compound semiconductors according to the known crystal pulling methods is extremely difficult.
Single crystals of GaAs are in increasing demand because they exhibit larger electron transfer than silicon (Si) when compared on a size or dimensional basis. Presently, the method used to obtain such GaAs single crystal is the well known horizontal Bridgman method. This method has the drawback that contamination may be caused by impurities diffused or transferred to the GaAs as the single crystal made is being supported by a quartz board or a quartz sealed tube. Also, according to this known method, the sectional shape of a grown crystal is not circular and this is a disadvantage. Recently, because of these technical problems and difficulties, various efforts have been made to try to grow such single crystals according to the Czochralski method.
According to the well known Czochralski method, in principle, a single crystal of a higher purity can be obtained than what is obtainable according to the horizontal Bridgman method. In the case of a GaAs in particular, and also GaP and InP, a technical problem still exists concerning the perfectness of crystals obtainable. Thus, research is continuously being made searching for a way to obtain crystals of such materials characterized by a smaller dislocation density.
Those skilled in this art well know and appreciate that the presence of excessive dislocations or a large dislocation density cause the electrical and optical characteristics of a semiconductor device made from these single crystals to be deteriorated or to show an abnormality
Such dislocations are sometimes created during the device manufacturing process, but in most cases are present from the beginning in GaAs single crystal used as substrate for manufacturing a device. The dislocations present in the substrate are caused mainly by a heat distortion during production of the single crystal, and this heat distortion is sometimes caused by an abrupt temperature gradient in the interior of the crystal induced by a convection current of an inert gas which is being used to establish an inert atmosphere of a high pressure in the crystal growing furnace. One major cause of dislocations is considered to be any non-uniform temperature distribution at the solid-melt interface caused by a heat convection current which occurs in the feed melt.
To prevent such heat convection, it is already known to float a heat convection preventing plate in a position below the surface of the feed melt when pulling a single crystal, the heat convection preventing plate having a diameter somewhat larger than that of the single crystal being grown but fairly smaller than that of the crucible containing the feed melt. This technique is effective against a heat convection current travelling from the center toward the outside in the feed melt, but is not always fully effective against a heat convection current travelling from the outside toward the center.
Further, it is known to float a convection preventing plate in a position below the surface of a feed melt by virtue of its buoyancy, the convection preventing plate having a diameter larger than that of the single crystal being pulled and somewhat smaller than the inside diameter of the crucible, and to conduct the feed melt upwardly beyond the convection preventing plate through fine holes formed in the plate or through a gap formed between the outer periphery of the convection preventing plate and the inner periphery of the crucible. But, when the feed melt is viscous, it does not pass smoothly through the fine holes and/or the gap, thus, slowing down the pulling operation and thereby increasing the time required to grow a crystal. Also, if carelessness results in a deficiency of the feed melt in the single crystal pulling region, this would cause great difficulties.
Further causes of dislocation are heat radiation from a liquid sealing agent such a B.sub.2 O.sub.3 which is often used to cover the upper surface of a feed melt, and heating differentials resulting from lowering of the feed melt level in the crucible as the crystal growth proceeds and are especially evident at the end of the single crystal pulling operation when the conditions are substantially altered from those at the beginning of the operation. The crucible plays the role of a heat retaining cylinder as the single crystal pulling operation progresses, thus making effective temperature control (control of a constant temperature gradient) extremely difficult.
At present, the internal temperature of a crucible containing feed melt is merely estimated by a thermocouple disposed outside the crucible. But from the above discussion, one can readily appreciate that such an arrangement is a coarse control and quite insufficient and unsatisfactory as a temperature measuring means for pulling a single crystal of a compound semiconductor as noted above. In such a case, the pulling operation requires the prevention of temperature variations which cause a heat distortion and providing and making an exact control.